This invention relates to a mark detection apparatus for a micro-roll film, and particularly to a mark detection apparatus which can be used for a micro-roll film having marks of different sizes.
A microreader and a reader-printer are the apparatus which projects an image of a microfilm on a screen or a photosensitive member. When a roll-like microfilm is used, the apparatus may be used in combination with a retrieval device. The retrieval device effects retrieval by counting marks (which are called blip marks) placed adjacent to each frame of the microfilm.
In one method of placing blip marks, a plurality of blip marks having different sizes are used on a single roll of film (hereinafter called "multi-blip system"). FIG. 1 shows a film 1 having blip marks L, M and S of sizes, large, middle and small. These blip marks L, M and S are placed in such a manner that the contents of the frames 2 contained in the film are classified into large, middle and small classes, in that a large-size mark L is placed on the head frame of a series of frames defined by the large class, a middle-size mark M on the head frame of a series of frames defined by the middle class, and a small-size mark S on the remaining frames. Then, for example, numbers (p, q, and r) are assigned to the marks (L, M, and S). The p-number mark L is first retrieved, the q-number mark M is then retrieved and finally the r-number mark S is retrieved thereby finding out a frame corresponding to the numbers (p, q and r).
The conventional device for classifying and detecting the blip marks of different sizes has a plurality of mark detection elements d1 . . . d4 arrayed in a predetermined spaced relation corresponding to the sizes of marks, as shown in FIG. 2. The on and off states of the detection elements are used to discriminate the mark sizes. However, this conventional device has a drawback of increasing cost since the device requires many detection elements. This device has a further significant drawback in that as the spacing between the detection elements is set in accordance with a certain rule, marks in accordance with a different rule cannot be detected. This results from the circumstances that there exists no unified standards for the sizes of blip marks, and blip marks available from the world have a variety of sizes.
Among the multi-blip systems, there is a film which uses marks of two sizes. However, if an attempt is made to retrieve this film by the aforementioned three-size detection apparatus, it has been difficult for the apparatus to discriminate, which one of three combinations, of blip sizes, large and middle, large and small, and middle and small, is used.
The present applicant has proposed an arrangement wherein a single detection element and a pulse train proportional to the travel of a film are used to discriminate blip marks having a plurality of sizes, as disclosed in U.S. patent application, Ser. No. 726,874, filed Apr. 25, 1985. According to this proposal, the number of pulses generated during the detection of marks by the detection element are counted, and the counted value is compared with a predetermined reference value to discriminate the sizes. In this proposal, the number of detection elements may be reduced but the reference value has to be predetermined. Thus, it was not possible to discriminate blip marks having plural sizes different from the aforesaid reference value.